The present invention relates to a high yield DFB laser with effective reject selection using criteria on threshold current.
In many applications of fibre optics one need good light sources. Desirable properties are high output power, good spectral behaviour (narrow bandwidth), high power efficiency, reasonable driving conditions, high yield, simple testing etc. One commonly used laser type, which to a great extent fulfills these requirements, is the DFB-laser. Traditionally, one use quarter-wavelength-shifted DFB-lasers with AR-coating (Anti Reflective coating) at both mirrors. As a complement or substitute to AR-coating sometimes so-called window structures are used. A window structure is a means to reduce the reflectivity of for example a laser. It is simply realised by termination of the laser waveguide a distance (e.g. 15 microns) from the chip facet. The light will then propagate freely in the bulk material between the waveguide termination and the chip facet. The beam will here diverge and the reflected beam as reflected at the chip facet will continue to diverge. Therefore the reflection coupled back into the laser waveguide will be substantially depressed compared with the case where the waveguide terminates at the chip facet. The quarter-wavelength-shifted DFB-lasers are equipped with a phaseshift of 90 degrees at the midpoint of the laser length. This laser type will in principle give 100% Single-Mode (SM) yield. The SM yield expresses the percentage of fabricated lasers that fulfill a certain spectral-purity requirement. Often this requirement is expressed in the property gain margin, which is the distance in net gain between the lasing mode and the next mode (closest to lasing among the others). However, this laser type will get half of the power in the forward direction and the other half in the backward direction i.e. it will not give any power asymmetry. This means that often these lasers give to little useful output power. Also this laser type gives relatively high threshold currents. To improve these drawbacks one often leave the back-mirror uncoated, with the natural reflectivity, which is typically in the order of 30%. This will give a laser with more power in the forward direction than in the backward direction. Further this will give lower threshold currents than for traditional quarter-wavelength-shifted DFB-lasers. However, when the back mirror is left uncoated the SM-yield will drop for the laser. This means that one has to select the good lasers among the lasers in the fabricated laser population.
As mentioned above, the threshold current and power asymmetry performances are not good enough for the quarter-wavelength-shifted laser for many applications. For the quarter-wavelength-shifted laser with as-cleaved backmirror mentioned above, the gain-margin-performance is not as good as desired and the need to select lasers by measuring optical spectra makes testing more complicated than what is desired. Also, the lasers that have gain margins that are good enough will have an undesirable spread in power asymmetry among themselves.
As indicated above, the demand for high output power often will require use of lasers with asymmetric power distributions with most of the light power coming out from the front mirror. One good way to accomplish this is to use lasers with relatively high rear-mirror reflectivity usually obtained by leaving the rear mirror as-cleaved, i.e. without AR-coating. However, by using this type of laser one can not accept all fabricated lasers. Due to the varying phase shifts at the mirrors, some of the lasers will not work with good enough spectral properties and must be sorted out. This sorting is conventionally performed using measurements of the optical spectrum of the laser. This is a relatively complicated measurement. Also, when using conventional lasers some of the samples, which must be thrown away, have among the lowest threshold currents and the best power uniformity in the population, which is an undesirable situation.
The laser of this application, which typically has an as cleaved back mirror, an AR-coated front mirror and a 90 degree phase shift (wavelength/4) positioned at 43% of the laser length counted from the rear mirror, gives good SM-yield, advantageous power asymmetry, and possibility to select the good lasers using the threshold current as selection parameter.